tmux attach behaves differently than tmux

session

wwalker@polonium:~ ✓ $ echo $TERM
xterm-256color
wwalker@polonium:~ ✓ $ pgrep tmux
wwalker@polonium:~ ✘ $ tmux attach
[exited]
wwalker@polonium:~ ✓ $ # TERM in tmux was screen
wwalker@polonium:~ ✓ $ pgrep tmux
wwalker@polonium:~ ✘ $ tmux
[exited]
wwalker@polonium:~ ✓ $ # TERM in tmux was screen-256color
wwalker@polonium:~ ✓ $ grep term .tmux.conf
set-option -g xterm-keys on
set -g default-terminal "screen-256color"
wwalker@polonium:~ ✓ $